The object of this research is to determine the extent to which mammalian cells repair or tolerate damage to their DNA following treatment with mutagenic and/or carcinogenic agents at different phases of the cell cycle. Two mechanisms for handling DNA damage are known. Excision repair involves the removal of the damaged bases and resynthesis of the DNA strand (repair replication). Recombination repair occurs during S phase and results in a bypass of the damaged region. The extent of these two processes following different mutagenic treatments is unknown. Using agents that produce different known types of lesions in DNA we intend to follow the loss of such lesions in the absence (during G1) or presence (during S) of DNA replication. This process is assumed to be associated with repair replication and this will be assayed at different phases of the cell cycle. In addition, by comparing replicated and non-replicated DNA it will be possible to determine if this is a prerequisite for DNA replication. In this case we intend to test two possibilities: DNA replication may be interrupted; or excision repair may be part of the replication process and enhanced at the DNA growing fork. This project involves the use of synchrony techniques, estimations of rate of DNA replication, quantitation of repair replication, isolation of the DNA growing fork and the detection of lesions. The latter include UV-induced thymine dimers, DNA single-strand breaks, alkylation of bases and cross-linking of the DNA strands.